The documents show links to 72 current or former heads of state in the data, including the Icelandic Prime Minister, Sigmundur Gunnlaugson, who had an undeclared interest linked to his wife's wealth and is now facing calls for his resignation.

The files also reveal a suspected billion-dollar money laundering ring involving close associates of President Putin.

Gerard Ryle, director of the International Consortium of Investigative Journalists (ICIJ), said the documents covered the day-to-day business at Mossack Fonseca over the past 40 years.

"I think the leak will prove to be probably the biggest blow the offshore world has ever taken because of the extent of the documents," he said.

Eleven million documents held by the Panama-based law firm Mossack Fonseca have been passed to German newspaper Suddeutsche Zeitung, which then shared them with the International Consortium of Investigative Journalists. BBC Panorama is among 107 media organisations - including UK newspaper the Guardian - in 78 countries which have been analysing the documents. The BBC doesn't know the identity of the source

They show how the company has helped clients launder money, dodge sanctions and evade tax

Mossack Fonseca says it has operated beyond reproach for 40 years and never been accused or charged with criminal wrong-doing

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The documents also shed light on how Mossack Fonseca offered financial services designed to help business clients hide their wealth.

One wealthy client, American millionaire and life coach Marianna Olszewski, was offered fake ownership records to hide money from the authorities. This is in direct breach of international regulations designed to stop money laundering and tax evasion.

An email from a Mossack executive to Ms Olszewski in January 2009 explains how she could deceive the bank: "We may use a natural person who will act as the beneficial owner… and therefore his name will be disclosed to the bank. Since this is a very sensitive matter, fees are quite high."

Ms Olszewski did not respond to the BBC's questions.

In a statement Mossack Fonseca said: "Your allegations that we provide structures supposedly designed to hide the identity of the real owners, are completely unsupported and false.

"We do not provide beneficiary services to deceive banks. It is difficult, not to say impossible, not to provide banks with the identity of final beneficiaries and the origin of funds."

The data also contains secret offshore companies linked to the families and associates of Egypt's former president Hosni Mubarak, Libya's former leader Muammar Gaddafi and Syria's president Bashar al-Assad.

Russian connection

It also reveals a suspected billion-dollar money laundering ring that was run by a Russian bank and involved close associates of President Putin.

The operation was run by Bank Rossiya, which is subject to US and EU sanctions following Russia's annexation of Crimea.

The documents reveal for the first time how the bank operates.

Money has been channelled through offshore companies, two of which were officially owned by one of the Russian president's closest friends.

Concert cellist Sergei Roldugin has known Vladimir Putin since they were teenagers and is godfather to the president's daughter Maria.

On paper, Mr Roldugin has personally made hundreds of millions of dollars in profits from suspicious deals.

But documents from Mr Roldugin's companies state that: "The company is a corporate screen established principally to protect the identity and confidentiality of the ultimate beneficial owner of the company."

Iceland connection

Mossack Fonseca data also shows how Icelandic Prime Minister Sigmundur Gunnlaugsson had an undeclared interest in his country's bailed-out banks.

Mr Gunnlaugsson has been accused of hiding millions of dollars of investments in his country's banks behind a secretive offshore company.

Leaked documents show that Sigmundur Gunnlaugsson and his wife bought offshore company Wintris in 2007.

He did not declare an interest in the company when entering parliament in 2009. He sold his 50% of Wintris to his wife for $1 (70p), eight months later.

Mr Gunnlaugsson is now facing calls for his resignation. He says he has not broken any rules, and his wife did not benefit financially from his decisions.

The offshore company was used to invest millions of dollars of inherited money, according to a document signed by Mr Gunnlaugsson's wife Anna Sigurlaug Pálsdóttir in 2015.

"If we detect suspicious activity or misconduct, we are quick to report it to the authorities," it said. "Similarly, when authorities approach us with evidence of possible misconduct, we always cooperate fully with them."